From the view point of Comparative cognitive science, we studied the evolution of intelligence examining various aspects of cognitive functions in humans, chimpanzees, macaques, and pigeons. The research topics in chimpanzees are as follows : Acquisition of linguistic skills using visual symbols such as lexigrams and Kanji characters, the concept of number memory retention process, visual information processing using a visual search paradigm, unity perception with occluders, the study of comunication using the task of simultaneous lever-pressing, crossmodal matching of auditory and visual stimuli, choice behavior between two discrimination tasks, computer-guided line drawing. In addition to these experiments in a single-subject test situation, we developed a face-to-face test situation where a human tester is directly interacting with the chimpanzee subject in a same room. The topics is the imitation of actions and the play with clay. In the third paradigm, the chimpanzee intelligence was explored in the outdoor setting. The emergence of tool use and the propagation was studied in the open-air compound where a group of 10 chimpanzee live as a community. Social interaction was also examined in a competitive feeding situation. The study as a whole was collaborated with the foreign researchers invited to the institute. The research team also sent the members to foreign countries, such as the research of a group of chimpanzees in Holland and the field work in Guinea in West Africa. The studies coming from the same idea is also partially replicated in a gibbon, macaques, capuchins and pigeons. All the studies were put together to illustrate the nature of intelligence and clarify the evolutionary trend.